Today, neurogenic pain, also referred to as inflammatory pain, can be defined as the consequence of a complex multi-factorial mechanism localized in the innervated tissues and/or the miniaturized endoneurial compartment in which, in addition to the nociceptive nerve terminals and/or the axons composing the peripheral sensitive fibers, the endothelial cells of the microcirculation, both tissue and endoneurial mastocytes, and cells of an immunologic nature extravasated from the tissue microcirculation in cases of suffering of the peripheral nervous system and/or of the tissue it innervates are involved. The whole process is defined as neurogenic inflammation, and it gives rise to the phenomenon referred to as peripheral sensitization.
The neuropathic pain, characteristic of neuropathies caused by traumatic, compressive, dysmetabolic, and infective injuries, is characterized by spontaneous pain, allodynia, and hyperalgesia. Today, the “central sensitization” found at the level of the dorsal horns of the spinal cord, generally as a consequence of a lesion or an alteration of the somatosensory nervous system (Neurology 2008; 70:1630-1635), is recognized as the most important mechanism on which of chronic pain is based. In addition to the synaptic junction between the first and second neuron, glial cells and in particular the microglia are involved in this sensitization process. The activation and proliferation of the microglia induced, as stated above, by injury or dysmetabolism of neuronal systems (peripheral, spinal, supraspinal) results in a significant alteration of the synaptic plasticity induced by growth factors of protein origin released by the microglia, with substantial modification of the neurotransmission (peptidergic, glutamatergic) dialogue between the first and second neurons. In particular, the neurotransmission at the level of said synaptic junction is deeply changed, passing from an essentially glutamatergic neurotransmission through NMDA-type receptors to a mixed neurotransmission, glutamatergic—in part still of an NMDA type and in part of a metabotropic type (through a series of glutamatergic receptors belonging to the mGlu family)—and peptidergic, essentially related to the neuropeptide knowns as Substance P (SP).
Since time, it has been known that palmitoylethanolamide, an endogenous lipid of an N-acylethanolamide nature produced on demand in the case of a cell damage, is capable of modulating in an inhibitory manner both the hyperdegranulation of the mastocyte and the hyper-activation of the microglia, thus showing to be capable, when administered in such a pharmaceutical form as to ensure bioavailability at the level of the above-mentioned two target cells, controlling the neuronal—peripheral and central—sensitization and, consequently, both the inflammatory and neuropathic pain. In particular, palmitoylethanolamide showed a particularly relevant effect, at the dose of 10 mg/kg, in the neuropathic pain model obtained by sciatic nerve ligation in the mouse; clinically, many studies proved the ability of palmitoylethanolamide, administered p.o. in an appropriate form—e.g., micronized and/or ultra-micronized—of reducing both inflammatory pain and chronic and neuropathic pain associated with several disease conditions.
L-Acetylcarnitine (LAC), a molecule which is typically used in the treatment of painful neuropathies, proved to be able, at a dose of 100 mg/kg, to decrease the mechanical allodynia in the neuropathic pain model obtained by sciatic nerve ligation in rodents (CCI). The mechanism of action of such molecule is a selective over-regulation of the metabotropic receptors for glutamate, and in particular of the receptor referred to as mGlu2 at the level of the joint between the first and the second neurons, located in the posterior horns of the spinal cord.